It's a fact that RT does very commonly result in bladder and bowel issues. Bladder problems (increased urgency to pee) generally peak about a fortnight after RT end and take about a month to settle down. Bowel issues last longer; mine had largely settled down after 6 months, but took a year to return to anything like normal.
With surgery, on the other hand, you have almost inevitable, although usually temporary, incontinence. Not to mention the fact that you're not going to be able to do the strenuous walking that being a postman presumably entails for probably 2-3 months after surgery.
Whichever treatment you have, you should probably plan to be off worth for a couple of months minimum.
All the best,